High vacuum system with recirculating conduit



April 28, 1953 R. H. MOFEE 2,636,655

HIGH VACUUM SYSTEM WITH RECIRCULATING CONDUIT Filed Dec. 15, 1948[river-110T RAYMOND H. MCFEE BY E22? Patented Apr. 28, 1953 HIGH VACUUMSYSTEM WITH RECIRCULATING CONDUIT Raymond H. McFee, Belmont, Mass,assignor to Photoswitch, Incorporated, Cambridge, Mass, a. corporationof Massachusetts Application December 15, 1948, Serial No. 65,400

7 Claims.

This invention relates to high vacuum pumping systems and particularlyto the introduction of controlled quantities of gases to such systemsfor the manufacture and testing of electron tubes and other types ofapparatus requiring high vacuum processing.

A conventional pumping system for producing a high vacuum consistsgenerally of two pumping stages. A mechanical fore pump is used toproduce a vacuum, referred to as the fore vacuum in a closed chamber,usually a glass flask, referred to as a ballast flask. The high vacuumstage consists of one or more mercury or oil diffusion pumps whichexhaust the remaining air from the high vacuum end of the system intothe fore vacuum chamber. Leading from the high vacuum side of thediffusion pumps a glass manifold, which may have several branches, andto which the vacuum tubes, or other devices to be pumped out, arehermetically sealed. Gauges of suitable types are connected at variouspoints in the system for measuring pressure. The system is also providedat various points with traps to eliminate condensible vapors.

When it is necessary to introduce a gas into the high vacuum end of thesystem, for example, to fill vacuum tubes with an inert gas, or to bringabout various chemical reactions in the sensitis ing of cathodematerials, a static" method is generally used in the prior art. Thepressure is first pumped down to the desired level. The diffusion pumpsare then shut down, and a metered quantity of gas admitted from anyavailable source to the fore vacuum end of the system at any convenientpoint, for example, through a tap leading into the ballast flask. Withthe diffusion pumps shut off, the gas pervades the entire system at thefore vacuum pressure. When this method is used, an objectionably longperiod of time is required before the diffusion pumps cease pumpingcompletely. Furthermore, if it is desired to recycle between evacuatedand gas pressure conditions, a period of fifteen or twenty minutes isusually required before a high vacuum can be again produced in thedevice being processed.

An alternative method is to leak gas continuously into the fore vacuumend of the system with the fore pump running. This method involvesconsiderable waste since a large percentage of the gas is immediatelyremoved through the fore pump and escapes to the atmosphere.Manufacturing operations which involve chemical reactions at constantlow gas pressure present special diiiiculties, since the loss of gas byreaction tends to change the pressure in the system.

If the volume of gas admitted is made so large that the loss by reactionis negligible, the waste may become excessive. At best, the pressurecannot be accurately controlled or maintained.

An object of this invention is to provide an economical and readilycontrollable means for supplying gas at an accurate, constant lowpressure to the high vacuum end of a pumping system of the type hereindescribed.

Another object of this invention is to provide a method and apparatusfor quickly recycling between evacuation and gaseous pressure conditionsin high vacuum systems.

In general, the desired result is here achieved by connecting the highvacuum end of the system to the fore vacuum by means of a conduit ofrelatively high impedance to gas flow so that a certainamount ofrecirculation takes place from the fore vacuum end to the high vacuumend. By this method it is possible to perform processing operations withvarious gases for a considerable period of time without shutting downthe high vacuum pumps, and also to recycle quickly between evacuationand gas conditions.

In the drawings illustrating the invention:

Fig. l is a schematic diagram of a conventional high vacuum pumpingsystem modified in accordance with the present invention by providing aconduit of relatively high impedance 'to gas flow betweenthe high vacuumend of the system and the fore vacuum;

Fig. 2 is a schematic diagram of an alternate type of valve used tocontrol the gas flow;

In Fig. l a fore vacuum pump 1 of suitable mechanical or otherconventional type, is illustrated as connected by manifold 2 to the forevacuum ballast flask 3. According to usual practice, the volume of theflask 3 is large enough to cushion the remainder of the system againstpressure fluctuations produced by the fore pump. Once flask 3 has beenexhausted to the minimum pressure which the fore pump is capable ofproducing, pressure inside the flask remains substantially constant aslong as the fore pump is kept running, and thus provides a constantrelatively low pressure chamber as an outlet for the high vacuum pumps.stopcock 24 provides for the isolation of fore vacuum pump I fromballast flask 3. A manifold 4 leads from flask 3 to a series of mercuryor oil diffusion pumps 5 and 6. The number and type of such pump u eddepends on the capacity of the system and the vacuum required, and,since their general opera tion and arrangement is well-known in the art,the pumps are not here illustrated in detail. The finaldittusion pump 6leads through a liquid air trap 22 of conventional construction, to ahigh vacuum manifold I, to which are connected the devices which are tobe evacuated, for example, an electron tube 23. A number of such devicesmay ordinarily be processed at the same time depending on the capacityof the system and the type of processing. A pressure gauge 8 of theconventional McLeod type is here illustrated as connected to the highvacuum manifold. The recirculating tube 9, which is here incorporatedinto the system for the purpose of admitting gas to the high vacuum end,is connected from the high vacuum manifold 1, preferably beyond trap 22,back to the fore vacuum ballast flask 3. The tube 9 is interrupted by acut-off valve within enclosure 2| of a conventional constructioncomprising a U-shaped bend It terminating in a vertical inlet tube IIwhich extends down into a closed reservoir [2 partly filled withmercury. An inlet tube l3, through which atmospheric pressure can beadmitted by opening stopcock It, is connected to the top of thereservoir so as to force mercury up through tube l I into U-bend It, andshut the recirculating tube.

A preferred method of operation of the apparatus illustrated in thedrawing to accomplish an evacuation-gas filling cycle for tube 23 is asfollows. Initially, stopcock I6 is closed, isolating mixing flask i5,and stopcock [4 of the cut-off valve within enclosure 2! is opened sothat the mercury within reservoir l2 rises to U-shaped bend [9 therebyobstructing gas flow through recirculating tube 9. With stopcocks i1 and24 open, the system is evacuated by operating pumps l, 5 and t in theconventional manner.

When envelope 23 has been evacuated to the desired extent, as read byMcLeod gauge 8, fore vacuum pump I is isolated from the system byclosing stopcock 24, and evacuated section i8 is also isolated byclosing stopcock ll. stopcock i6 is then opened, allowing the gas withinflask Hi to enter evacuated section It. stopcock I6 is thereafter closedisolating the gas within section it from the rest of the system.

With diffusion pumps 5 and 6 continually pumping, stopcock I1 is opened,allowing the gas within section Hi to diffuse into ballast flask 3,manifold 4, and the portion of conduit 9 to the left of U-shaped bendit. Stopcock I! is thereafter closed.

The admitted gas does not enter the high vacuum portion of the systemrepresented by manifold l and envelope 23 because diffusion pumps 5 and6 are continually pumping and recirculating tube 9 is closed off by thecut-off valve within enclosure 2!. Furthermore, this gas is not drawnoff because pump 1 is isolated by means of closed stopcock 24.

With diifusion pumps 5 and 6 running, the pressure diiferential existingbetween manifold l and flask 9 causes gas to be drawn into the highvacuum end of the system through tube 9 when stopcock M is closed,thereby opening the cut-off valve within enclosure 2|. Since thisdifferential is relatively constant for fixed sizes of the vacuum systemcomponents, a constant gas pressure will appear at the high vacuum endof the system which is determined in a large measure by the pressuregradient which can be built up across the tube length of recirculatingtube 9. In general, if tube 9 is relatively small in cross-sectionalarea, a larger pressure gradient will be developed across tube 9 whenvalve 2| is open, thereby causing the gas pressure within manifold 1 tobe relatively low as compared to that which would 4 be created by alarger cross-sectional area of tube 9.

The loss or absorption of gas by chemical reactions within the vacuumsystem are to a large extent compensated for or nullified by thecontinual recirculation of substantially the entire mass of admitted gasthroughout the high vacuum end of the system.

The admission of gas into the high vacuum end of the system can bestopped and started quickly and as desired by opening or closingstopcock l4 so that the cut-off valve within enclosure 2| is closed oropened. When tube 9 is obstructed after the introduction of gas intotube 23, pumps 5 and 6 quickly evacuate manifold 1 and attached tube 23.This gas is again stored within flask 3. Since circulation is maintainedin the system during the admission of gas, condensation products arereadily disposed of by conventional liquid air trap 22.

The arrangement here disclosed not only provides greater economy in theuse of gas and more accurate control of the pressure than does theconventional static method, previously described herein, but makeseconomical and practicable certain types of operations for which thestatic method is wholly unsuited, such as gas processing at low pressureover long periods of time. Furthermore, during the evacuation and gasprocessing steps of this invention, diffusion pumps 5 and t arecontinually operating. Therefore, the delays inherent in the prior arthigh vacuum processing techniques, wherein it is necessary to shut downand restart the diffusion pumps, are completely eliminated.

Fig. 2 illustrates a modified cut-off valve which provides for operationwithin two alternative ranges of gas pressure. A capillary 26 hereshunts the U-shaped bend H3. With this arrangement, the tube 9 can bepartially shut by filling the lower part of the u with mercury, orwholly shut by raising the mercury to cover the ends of the capillary.The ends of the capillary are preferably sloped downward to ensure thatthe mercury will run out readily when the valve is closed. It isapparent that a relatively high range of gas pressures may be maintainedin the high vacuum end when both paths through the valve are open, and alower range when the capillary alone is open. This type of valve isespecially suitable if the pumping systems are to be used formanufacturing diiferent kinds of devices, or where successive'gasprocessing operations are to be performed at different pressures.

Since certain changes may be made in the above-described article anddifferent embodiments of the invention could be made without departingfrom the scope thereof, it is intended that all matter contained in theabove description or shown in the accompanying drawings shall beinterpreted as illustrative only and not in a limiting sense.

What is claimed:

1. The method of supplying a gas to a high vacuum pumping system havingfore vacuum and high vacuum system portions interconnected by a highvacuum pump, which comprises admitting the gas to the fore vacuumportion of said system, transmitting the admitted gas to the high vacuumportion of said system through a conduit which shunts said high vacuumpump, and pumping by said pump the gas in the high vacuum portion ofsaid system back to the fore vacuum portion of said system, whereuponsaid gas is again circulated.

2 The method of claim 1 further characterized by continuous operation ofthe high vacuum pump during the admission of the gas.

3. A gaseous evacuation and filling system comprising: a fore pump; afore vacuum ballast chamber connected to the vacuum side of said pump; ahigh vacuum manifold adapted for connection to a device to be evacuatedand subjected to gas; a high vacuum pump connected between said manifoldand said chamber; means for admitting gas to said chamber; and arecirculating conduit interconnecting the high vacuum and the exhaustsides of said high vacuum pump whereby said admitted gas can be made tocirculate in a path which includes said chamber, conduit and high vacuumpump back to said chamber.

4. A system according to claim 3 wherein said conduit is provided with ashut-off valve.

5. A system according to claim 4 wherein said shut-off valve comprises aU-shaped tube and means for filling said U-tube to a desired level ofmercury.

6. A system according to claim 5 wherein the U-shaped tube of saidshut-off valve comprises a capillary interconnecting the legs of said U.

7. A high vacuum pumping system comprising: a fore pump; a fore vacuumballast chamber conneeted to said pump; a high vacuum manifold adaptedfor connection to a device to be evacuated; a diffusion pump connectedbetween said manifold and said chamber; means for admitting gas to saidchamber; a vapor trap connected between said manifold and said highvacuum pump; and a conduit of relatively high impedance to gas flowconnected from the manifold side of said trap to said chamber.

RAYMOND H. MCFEE.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,145,830 Espersen Jan. 31, 1939 2,307,754 Beckwith Jan. 12,1943 2,369,563 Gustin et a1. i Feb. 13, 1945 2,469,434 Hansen et al May10, 1949

